In itself, cathodic corrosion protection of a metal body, such as a ship's hull, surrounded by an electrolytic medium, such as seawater, by means of a metal anode having a lower quiescent potential, such as a sacrificial zinc electrode, is known. When a zinc anode is used for the protection of a coated steel ship's skin, this anode is, for example, electrically short-circuited with the ship's skin. With the known zinc anode, corrosion of a body made of a certain type of metal can often be inhibited in an adequate manner.
Besides this cathodic protection by means of a sacrificial electrode, which is known as a passive protection, there is a form of active protection according to which an anode is placed in the vicinity of the metal body to be protected and this anode is subsequently kept at such a potential that oxidizing reactions on the surface of the body to be protected are prevented.
A disadvantage of the known arrangements for cathodic protection is that they often promote disbonding of a synthetic or plastic coating applied to a body to be protected. It is known that such disbonding can be caused by hydroxide ions produced in cathodic reactions on the surface of the body to be protected.
In numerous practical situations there is a need for an arrangement for cathodic protection where any disbonding of a plastic attached to a metal to be protected does not occur. Moreover, in numerous practical situations there is a need for an arrangement for cathodic protection of a body comprising several types of metal which have different quiescent potentials in a particular medium. Sea-going vessels of which the hulls are made of a carbon steel are sometimes provided with specific underwater equipment. Such equipment is often accommodated in a stainless-steel housing which is necessarily provided with sealing interfaces to a synthetic material (plastic).
When such underwater equipment is freely exposed in seawater, various types of corrosion may occur, with especially crevice corrosion posing a threat to the equipment. Crevice corrosion is a form of local attack occurring as a result of differences in the oxygen content of the electrolytic medium, seawater in the present case, in crevices and cracks in regard to the medium at the greater part of the stainless-steel surface. In the crevice, a depletion of oxygen takes place and the protective oxide film built up in normal conditions and existing on the stainless steel is broken down locally in the crevice, while, in addition to this, a galvanic action takes place between the crevice and the remaining part of the stainless-steel surface. In the crevice, the pH becomes lower and lower by the release of H.sup.+ -ions, resulting in an increasingly fast progressing attack. Crevice corrosion may result in leakage.
I have found that an effective cathodic protection from corrosion of such a stainless-steel housing with sealing interfaces to a plastic material is not feasible by making use of a known arrangement. Especially, the development of disbonding of a plastic attached to stainless steel presents a problem.
The present invention relates to an arrangement for cathodic protection from corrosion of a metal body surrounded by an electrolytic medium, comprising a metal anode body which is to be introduced into the medium and has a lower (electric) quiescent potential in the medium than the body to be protected. The quiescent potential of a metal body surrounded by an electrolytic medium is understood to mean the potential of a body freely exposed in this medium, that is to say that the body is not part of an electric circuit: The value of the quiescent potential is usually stated with regard to a certain reference electrode.